1. Field of the Invention
The invention relates to a structure of a surface mounted resettable over-current protection device and a method for manufacturing the same, and in particular to a surface mounted resettable over-current protection device formed without using through holes and electroplating process and having five-conducting surface terminal electrodes, and a method for manufacturing the same.
2. Description of the Related Art
To prevent electronic systems from over-current damages caused by an abnormal condition, more and more electronic systems are provided with over-current protection devices. With such an provision, damages can be confined to the over-current protection devices when an over-current problem occurs in the electronic systems. A further concept is that costs for after-sale services and maintenance are greatly reduced if the protection devices can perform protection functions once over-current occurs and then they return to the normal condition. For these reasons, a fusible over-current protection device is gradually replaced with a polymer positive temperature coefficient (PPTC) material-based resettable over-current protection device which is widely used in various electronic systems. For high-density integration applications of the electronic systems, a resettable over-current protection device can be divided into a DIP type and a surface mounted type. Both types are used in packaging, wherein the growth rate of the need for the surface mounted type prevails over that of the DIP type.
A feature of a resettable over-current protection device is that when a current flowing through a polymer positive temperature coefficient material is over an upper limit, the temperature of the device rises to cause the original lowest resistance to increase rapidly so as to limit the current flow. A simplest polymer positive temperature coefficient material structure utilizes a polymer positive temperature coefficient material, and like a conventional two-sided printed circuit board (PCB), each of the two opposite sides of which is provided with a conducting metal foil. Therefore, the development of a prior surface mounted resettable over-current protection device is based on a printed circuit board process, wherein electrodes are formed by electroplating through holes of a substrate.
FIGS. 1–7 show a flow chart of manufacturing a conventional surface mounted resettable over-current protection device. Referring first to FIG. 1, a raw material substrate 100 having a polymer positive temperature coefficient material layer is provided. On each of the two opposite surfaces of the substrate 100, a conducting metal foil 102 is formed.
Next, referring to FIGS. 2 and 3, through holes 104 are formed using an automatic driller, and then, the inner walls of the holes are electroplated to form conducting layers 106 to thereby connect the conducting metal foils 102 on the two sides of the raw material substrate 100.
Referring to FIGS. 4 and 5, a plurality of trenches 107 are formed on the conducting metal foils 102 by photolithography and etching in the printed circuit board process so as to form bodies of surface mounted resettable over-current protective devices. After that, an insulating solder mask 108 is formed on the both side of main structures.
Finally, referring to FIGS. 6 and 7, the entire substrate 100 is cut into a plurality of surface mounted resettable over-current protection devices along cutting lines.
The terminal electrodes of the conventional surface mounted resettable over-current protection devices are mainly formed by through holes and electroplating processes. Basically, the conducting metal foils on the two sides of the substrate are connected to each other via the conducting layers formed on the inner walls of the through holes. Due to the limitation on the sizes of the electrodes, the diameters of the though holes are limited, resulting in an effect on the performance of the resistance of the terminal electrodes.
In a process for forming conventional surface mounted resettable over-current protection devices, the area of a polymer positive temperature coefficient raw material substrate can only be enlarged to a certain level, and there still is a great difference in area as compared with a substrate used in a real printed circuit board process. Therefore, completely using a printed circuit board process to manufacture a surface mounted resettable over-current protection device should take adjustments in process and economics into consideration.
Furthermore, since automatic drilling and through holes electroplating apparatuses are required to form the terminal electrodes of the surface mounted resettable over-current protection devices, it incurs more costs spent therefor. Meanwhile, for a new process, re-learning is necessary.
In view of the above, an object of the invention is to provide a structure of a surface mounted resettable over-current protection device and a method for manufacturing the same. The terminal electrodes of the device can be formed without using through holes and electroplating processes. The device can be efficiently and economically manufactured by a process for manufacturing a passive resistor terminal electrodes structure which is already used for mass production.